The present invention relates to a system for encoding and decoding digital video data which is divided into blocks of predetermined size. More particularly, the present invention relates to an encoding and decoding method and apparatus for reducing blocking artifact, which is a phenomenon of degrading the quality of a picture that is reproduced by dividing each frame of video data into a plurality of blocks and encoding the divided blocks.
Conventional systems for transmitting and receiving video and audio signals often employ an encoding and decoding system. Such encoding and decoding system encodes the video and audio signal into digital data, stores the data, and then transmits the digital data to a receiver. At the receiver, the encoded data is then decoded so as to be reproduced into the original signal (i.e., the signal prior to be encoded). Typically, the encoding of the data includes some form of data compression so that less data needs to be stored and transmitted.
FIG. 1A illustrates a conventional encoding system. Video data of each frame is divided into blocks of size N.times.N (which is generally represented as N.sub.1 .times.N.sub.2, but for the convenience of explanation, is assumed as N.sub.1 =N.sub.2 =N which represents a pixel unit). Each block of data is input to an orthogonal transformer 1. The orthogonal transformer 1 performs data-transformation, such as DCT (discrete cosine transform) with respect to each block data, and converts the block data into transformation coefficients of the frequency domain. The output of the orthogonal transformer 1 is then applied to a quantizer 3, which changes the conversion coefficients into representative values, each of which has a predetermined level, after taking the energy distribution of the transformation coefficients into consideration. A variable length encoder 5 further compresses the data by variable-length-encoding the representative values using statistical characteristics of the representative values.
The encoded, compressed data is then transmitted to the decoding system shown in FIG. 1B. The received data passes through restorage means having a variable length decoder 11, an inverse quantizer 12, and an inverse orthogonal transformer 13, which decode and uncompress the data to produce data close to the state before being encoded.
FIGS. 2A and 2B illustrate another example of a conventional encoding and decoding system, respectively. Generally, there are many similar portions (i.e., frames or blocks) of a picture. In this situation, the data can be further compressed by generating a motion vector, which is calculated by estimating the amount of motion between adjacent frames of a picture, and through differential-pulse-code-modulation (DPCM). In the decoding process, the motion vector which is calculated in the encoding procedure is used to compensate or reconstruct the block of data, thereby reproducing the original data.
An encoding and decoding system which utilizes the motion-compensated DPCM method includes a predetermined feedback loop for the motion compensation. As shown in FIG. 2A, the feedback loop for the motion compensation in the encoding system, includes an inverse quantizer 4, inverse orthogonal transformer 2, adder A2, frame memory 6, motion estimator 7, and motion compensator 8. The decoding system, as shown in FIG. 2B, similarly includes a feedback loop having a frame memory 14, motion compensator 15, and adder A3. Since such a DPCM process of the encoding and decoding system is known, the detailed description will be omitted.
Switches SW1 and SW2, which are provided in the respective apparatus of FIGS. 2A and 2B, refresh the video data on a frame or block unit basis to prevent accumulation of errors in the DPCM process. That is, when the switches SW1 and SW2 are turned on, the DPCM process is performed, while when the switches are turned off, the PCM data is encoded for transmission.
In such a conventional encoding and decoding system, since a single picture is divided into blocks of N.times.N size, and then the blocks are processed in order to encode and compress the video data, if the decoding system receives and reproduces the transmitted signal which has been encoded and compressed, the boundaries between blocks of the picture are easily exposed and, consequently, a blocking artifact of a certain portion appears as a lattice results. In other words, the boundaries of blocks are not capable of being easily identified which results in blocking artifacts occurring.
Recently, there have been several methods proposed to reduce such a blocking artifact. For example, in a first method, the divided blocks of the picture are overlapped with each other. A second method utilizes an overlapped orthogonal transform. In a third method, the boundaries of the blocks are passed through a low-pass filter(s) in the decoder. However, since the first and second methods change the basic composition of the encoding system, they are quite complex and require an extensive amount of additional hardware. The third method also has a problem in that the resolution of the boundary portion of the block is lowered.